1. Field of the Invention
This invention is concerned with the manufacture of olefins. It is particularly concerned with the catalytic conversion of an alcohol feed to a hydrocarbon mixture having a high content of light olefins.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. One such conversion, which has generated considerable interest, is the manufacture of olefins from alcohols and ethers.
U.S. Pat. No. 4,025,575 describes a process by which lower alcohols and/or their ethers are converted to a mixture of C.sub.2 to C.sub.5 olefins by contact at subatmospheric inlet partial pressure with a crystalline aluminosilicate zeolite catalyst having a constraing index of 1 to 12 and a silica to alumina ratio of at least 12.
The production of olefins from aliphatic ethers by catalytic conversion with, for example, an HZSM-5 zeolite catalyst is described in U.S. Pat. No. 3,894,106 issued July 8, 1975.
The use of diluents to dissipate exothermic heat in a two stage conversion of methanol to gasoline is described in U.S. Pat. No. 3,931,349 issued Jan. 9, 1976.
A two-stage conversion of a lower alcohol to olefins or to gasoline, which employs a tubular reactor for the second stage, is described in U.S. Pat. No. 4,058,576.
A process for the manufacture of ethylene by catalytic conversion of methanol in the presence of a substantially anhydrous diluent and a zeolite catalyst such as HZSM-5 is described in U.S. Pat. No. 4,083,888 issued Apr. 11, 1978.
The effect of the rate of feedstock flow past the zeolitic catalyst in a continuous reaction mechanism involving the conversion of methanol to hydrocarbons has been studied and reported in the scientific literature (C. D. Chang and A. J. Silvestri, Journal of Catalysis, 47 249 (1977)). The results, which are summarized in TABLE I, show that at a liquid hourly space velocity (LHSV) of 1, the selectively ratio of C.sub.2 -C.sub.4 olefins/paraffins is 2.8/41.1 or 0.068. By increasing the LHSV of the methanol feed to 108 and 1,080, however, this ratio changes to 5.28 and 4.95, respectively, indicating a preference for the formation of olefins over paraffins at higher space velocities. The overall conversion to hydrocarbons, however, is reduced significantly as the velocity is increased (100% at LHSV=1; 48% at LHSV=108; 9% at LHSV=1,080). Obviously, production costs would be significantly higher at the low conversion/high space velocity conditions than would the costs of a process where near 100% conversion of methanol to hydrocarbons could be realized.
TABLE I ______________________________________ EFFECT OF SPACE VELOCITY ON METHANOL CONVERSION AND HYDROCARBON DISTRIBUTION ______________________________________ LHSV 1 108 1080 [vol of liquid methanol/ (vol of catalyst/hr)] Product distribution (wt %) Water 56.0 33.0 8.9 Methanol 0.0 21.4 67.4 Dimethyl ether 0.0 31.0 23.5 Hydrocarbons 44.0 14.6 0.2 Conversion 100.0 47.5 9.1 (MeOH + MeOMe) (wt %) Hydrocarbon distribution (wt %) Methane 1.1 1.1 1.5 Ethane 0.6 0.1 -- Ethylene 0.5 12.4 18.1 Propane 16.2 2.5 2.0 Propylene 1.0 26.7 48.2 i-Butane 18.7 6.5 13.8 n-Butane 5.6 1.3 -- Butenes 1.3 15.8 11.9 C.sub.3 + Aliphatics 14.0 27.0 4.4 Aromatics 41.1 6.6 -- ______________________________________
Furthermore, the conversion of methanol to hydrocarbons is an exothermic reaction, evolving approximately 700 Btu of heat for each pound of reactant. Removal of such heat of reaction is a major problem, particularly in larger sized catalyst beds, and limits the methanol feed rate to low space velocities. Although high olefin/paraffin ratios can be obtained at high space velocities, heat removal and the need to separate, purify and recycle unreacted methanol at low conversions have made this route commercially impractical.